Acoustic interrogation of subsurface features tend to be limited by the size and power of practical sources, and in practice, the output of downhole acoustic transducers is limited by the power transmission capabilities of the wire line cable. High frequency signals have a relatively short penetration distance, while low frequency signals generally require large sources, clamped to the borehole wall, to maximize energy transfer to the formation and minimize unwanted signals within the well bore. Attempts have been made to produce such a low frequency signal, but each has had its own shortcomings. For example, U.S. Pat. No. 5,521,882 by D'Angelo et al. discloses an acoustic tool designed to record non-linear waves that propagate along the borehole wall and refract back into the well, with limited penetration onto the surrounding rock formation. U.S. Pat. No. 7,301,852 by Leggett, III et al. discloses using a bottom hole assembly having an array of transducers located in opposite ends of the assembly to produce the signals that converge at an angle, as reported by Johnson et al (1987). The signals from non-linear mixing at an angle are extremely weak and diffuse, and would not penetrate deep into the surrounding rock formation. In light of these prior attempts, there is a need for an acoustic signal source that can produce low frequency energy from the borehole in the subsurface environment.